# Find the switch

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We are in a two-story house. On the ground floor there are three switches and on the upper floor there is a single bulb. How can we find out which of the three switches is the one that turns on that bulb taking into account that we can only go up one time to the floor above to check if the bulb is on?

#### Solution

The solution is to turn on the first switch and wait a few minutes. We turn off the first switch, turn on the second and go up to check. If the bulb is on, the correct switch is obviously the second. If it is off, but we touch the bulb and it is hot, it means that it was turned on recently, so the correct switch is the first. And if finally the bulb is off and cold, it means that we have not turned it on with any of the first two switches, so the third one is correct.